Various embodiments are based on a method for operating a high-pressure discharge lamp outside the nominal power range thereof.
During the operation of discharge lamps, which are also called lamps for short hereinafter, there is the problem of the stable arc attachment of the discharge arc on the electrode tips. Under certain operating conditions, the discharge arc jumps from one arc attachment point to another. This jumping of the discharge point is also referred to as arc jumping and is manifested in lamp flickering. This is particularly disturbing if the light from the lamp is used for projecting images.
Projection devices such as video projectors often use so-called ultra short arc lamps on account of the prerequisites for the optical imaging. Said lamps are high-pressure discharge lamps which have a very short electrode spacing in order to be able to ensure a good optical imaging of the video projector. On account of the high power of these lamps and the short electrode spacing, the electrodes become very hot. Therefore, simple pin electrodes cannot be used in these types of lamps. Instead, use is made of electrodes having a very wide electrode head in order to increase their thermal mass. In this case, the head diameter is typically greater than the electrode spacing (e.g. head diameter of 1.5 mm in a lamp having an electrode spacing of 1.0 mm).
Hereinafter, the term electrode end denotes the inner end of the lamp electrode projecting into the discharge space of the gas discharge lamp burner. The term electrode tip denotes a needle- or lug-shaped elevation which is seated on the electrode end and the end of which serves as an attachment point for the arc.
EP 1 152 645 discloses a method which allows electrode tips to grow on the electrode by means of current pulses, also called maintenance pulses hereinafter. These grown electrode tips initially have the advantage that the plasma arc of the arc discharge generated in the lamp finds a stable attachment point on the electrode and does not jump between a plurality of attachment points. What is crucial in this case is the ability of the electrode to be able to supply a sufficiently high current, which depends crucially on the temperature of said electrode. If said temperature is too low, the tip of the electrode is not liquid, and the arc attachment is unsatisfactory on an electrode tip that is not at least partly liquid. An excessively cold tip leads to solidification of the liquid tungsten and the arc thereupon contracts, that is to say that a punctiform arc attachment occurs, because the energy density is increased as a result. However, said punctiform arc attachment is unstable and readily moves over the electrode tip, which can be perceived as flicker in the application. Moreover, a moving arc attachment, on account of the high energy density, leads to undesired changes in the front region of the electrode head. Video projectors often require a light source which has a temporal sequence of different colors. As is described in the document U.S. Pat. No. 5,917,558 (Stanton), this can be achieved by means of a rotating color wheel, for example, which filters changing colors from the light of the lamp. The time durations during which the light assumes a specific color need not necessarily be identical. Rather, a desired color temperature that arises for the projected light can be set by means of the ratio of said time durations with respect to one another.
The lamp is usually operated with a rectangular lamp current. The lamp frequency is understood to be the reciprocal of the period duration of the rectangular lamp current, as illustrated in FIG. 1. The lamp frequency at nominal power is understood to be the reciprocal of the period duration of the rectangular lamp current during operation of the lamp with nominal power. The nominal power is the power with which the lamp should be operated, as specified by the lamp manufacturer. At nominal power, the high-pressure discharge lamp is usually operated with a predetermined frequency. In the prior art, the lamp current is generated from a DC source with the aid of a commutation device. The commutation device usually consists of electronic switches which commutate the polarity of the DC source with the timing of the rectangular lamp current. During commutation, overshoots cannot be completely avoided in practice. Therefore, in the prior art the point in time at which a commutation is intended to take place is combined with the point in time at which the color of the light changes in order to mask out the overshoots. For this purpose, a sync signal is provided, which includes a sync pulse synchronously with the abovementioned color wheel. The color change and the commutation of the lamp current are synchronized with the aid of the sync signal. In advanced projection systems, the lamp current need not always exhibit a rectangular shape, rather the current level can proceed in a plurality of steps. This current profile over time is also designated hereinafter as the “waveform”. An explanation of the term will be found further below.
During the operation of discharge lamps there is the phenomenon of the growth of electrode tips which, as explained above, represent an essential prerequisite for a stable arc attachment. Material which evaporates from the electrodes at a location is deposited again at preferred locations on the electrode and can in this case contribute to the formation of electrode tips. Furthermore, as a result of the repeated melting and solidification of the tungsten at the electrode tip, tungsten material from electrode regions situated further back is transported into the tip of the electrode. These transport phenomena are greatly dependent on the temperature of the electrode, and also the changes in said temperature with respect to time and hence the operating mode of the lamp. The growth of the electrode tips can be caused e.g. by so-called “maintenance pulses”, which are also designated as commutation pulses hereinafter. These are short current pulses, usually shortly before the commutation, which have an increased current magnitude.
FIG. 1 shows an example of such a commutation pulse in a very simple waveform. The waveform is divided into a plateau and the commutation pulse. The plateau is described by a plateau length and a plateau height i.e. by a specific residence time of a current magnitude. The commutation pulse is likewise described by a pulse length and a pulse height i.e. by the duration of the pulse at a specific current magnitude. The commutation pulse provides for greater melting of the electrode in the front region, which is then contracted by the surface tension of the tungsten and subsequently cools again after the commutation pulse and the subsequent commutation. If this method is repeated at corresponding time intervals, a tip slowly forms from this. In this case, the commutation pulse should always precede the commutation for an effective application.
FIG. 2A shows a further example of a waveform having a further current boost alongside the commutation pulse. In this case, the period duration of the successive full cycles is always of the same magnitude. FIG. 2B shows a third example of a waveform of an advanced operating method, in which the period duration changes from full cycle to full cycle and the current waveform also changes from half-cycle to half-cycle. In such cases, the current profile is more complex and exhibits current boosts and staircase profiles which are synchronized with the sequence of the individual color segments of the color wheel. In the case of such complex current waveforms it is more difficult to operate the lamp optimally; for this purpose, it is necessary to observe some fundamental design rules when generating a waveform.
For stable and flicker-free operation, the temperature of the electrode should always be in a specific range, such that the electrode tip is indeed just liquid. The electrode tip is thus at the optimum temperature for a stable arc attachment. This is unproblematic in principle during operation of the lamp at nominal power and can be implemented with the known operating methods. However, if the lamp is intended to be greatly dimmed, that is to say operated at a power significantly lower than the nominal power, then the problem arises that the temperature of the electrodes decreases on account of the reduced lamp power, and flickering of the discharge arc occurs on account of the low temperature of the electrodes. If the lamp is intended to be operated with higher power, then the problem arises that the electrodes can become too hot and an increased electrode burn-back occurs. Furthermore, the increased temperatures compared with normal operation can result in denitrification of the burner vessel.